1. Field of the Invention
The present invention relates to a receiver system of an image signal from a weather satellite in which an image signal transmitted from a weather satellite is optimally converted into a lightness signal.
2. Related Background Art
In a conventional receiver system of an image signal from a weather satellite, as shown in FIG. 8, an amplitude-frequency modulated image signal (hereinafter, referred to as a transmission image signal or FAX-facsimile signal) which is sent from a weather satellite 1 is received by a parabola antenna 2. The received signal is converted by a down converter 3 into a frequency signal which can be easily process. This frequency signal is FM detected by an FM detector 4 and amplified by a pre-amplifier 5. Thereafter, the amplified signal is AM detected by an AM detector 6. The AM detected output is applied to a DC amplifier 8 through a low pass filter 7 to remove carrier signal and is amplified to a voltage level necessary for A/D conversion. On the other hand, the clock signal generated from a clock signal oscillator 11 is frequency divided by a frequency divider 12.
An output from the DC amplifier 8 which represents quantized levels of the temperatures or lightness of a cloud image is sampled and held by a sample/hold (S/H) circuit 9. An output of the S/H circuit 9 is converted into digital data by an analog/digital converter (hereinafter, referred to as an A/D converter) 10. An output of the frequency divider 12 is supplied as a sampling clock signal and a conversion clock signal to the S/H circuit 9 and A/D converter 10.
The output data from the A/D converter 10 is supplied as address data to a memory 13 and converted into lightness data stored therein. This lightness data is stored into a video RAM 14 and an image is displayed by a display 15.